The invention relates to a telephone branch line transmission circuit comprising a control device adapted to at least supply dial pulses and to control at least a first and a second switching device arranged between a first and a second branch line, and comprising a blocking capacitor coupled to the first branch line.
EP-A2-0,371,558, which corresponds to U.S. Pat. No. 5,121,425, issued Jun. 9, 1992, assigned to the present assignee, discloses a transmission circuit of the aforesaid type in which a series arrangement of two switching devices is arranged between two branch lines in parallel with the transmission direction. A blocking capacitor is connected to one branch line in order to block the flow of direct current. The blocking capacitor has its other terminal connected to a secondary terminal of a transformer whose primary terminals are connected, for example, to a terminal unit comprising a hybrid circuit, an amplifier and a telephone set. The secondary terminals of the transformer are coupled to a telephone exchange via the transmission circuit.
The control device, which comprises an optocoupler, supplies loop-closing and dial pulses to the switching devices. The first switching device comprises a MOS field-effect transistor and the second switching device comprises a bipolar transistor. The bipolar transistor is coupled to a voltage divider and a storage capacitor, the node between the two voltage-divider resistors being connected to one terminal of the storage capacitor and to the base of the bipolar transistor. One voltage-divider resistor has its other terminal connected to the control device. The bipolar transistor has its collector connected to one branch line and its emitter to the drain terminal of the MOS field-effect transistor of the first switching device. The field-effect transistor has its gate terminal coupled to a further voltage divider, which is connected to the control device, and its source terminal to a branch line.
Upon a loop-closing or dial pulse, the MOS field-effect transistor and the bipolar transistor allow the passage of direct current after a charging time of the storage capacitor. As a result of this, a loop is completed between the two branch lines.
A problem with such circuit is that in the cut-off state of the transistors of the two switching devices the exchange generally supplies a residual direct current, as a result of which the blocking capacitor is charged. Upon switching off, i.e. upon termination of the loop-closing or dial pulse, the transistors are turned off. However, that permits charging of the blocking capacitor by the residual direct current, which leads to distortions in the current supplied by the exchange in response to a dial pulse. As a result of this, the exchange may inaccurately detect the end of a dial pulse.